The death rate in Massachusetts dropped significantly after it adopted mandatory health care coverage in 2006, a new study found, offering evidence that the country’s first experiment with universal coverage — and the model for crucial parts of President Obama’s health care law — has saved lives, health economists say. Benjamin Sommers, assistant professor of medicine at Brigham and Women's Hospital, is the lead author. Katherine Baicker, professor of health economics at the Harvard School of Public Health, also participated in the study.